MapReduce code is written in Java. Greenplum provides Java APIs for use in the MapReduce
code. The Javadoc is available in the $GPHOME/docs directory. To view
the Javadoc, expand the file gnet-1.1-javadoc.tar and open
index.html. The Javadoc documents the following packages:

The HDFS cross-connect packages contain the Java library, which contains the packages
GPDBWritable, GPDBInputFormat, and
GPDBOutputFormat. The Java packages are available in
$GPHOME/lib/hadoop. Compile and run the MapReduce job with the
cross-connect package. For example, compile and run the MapReduce job with
gphd-1.0-gnet-1.0.0.1.jar if you use the Greenplum HD 1.0
distribution of Hadoop.

To make the Java library available to all Hadoop users, the Hadoop cluster administrator
should place the corresponding gphdfs connector jar in the
$HADOOP_HOME/lib directory and restart the job tracker. If this is
not done, a Hadoop user can still use the gphdfs connector jar; but
with the distributed cache technique.